A multilayer ceramic capacitor which utilizes, as the dielectric material between electrodes, a ceramic composition containing titanate such as barium titanate as the main component is one of essential parts for recent electronic devices for industrial or consumer use, because such a capacitor can have a small size and a large capacitance, excellent electric characteristics for a wide frequency band and good heat resistance, and can be easily mass-produced.
To manufacture such a multilayer ceramic capacitor, an organic binder, a plasticizer, a solvent and a dispersant, for example, are added and mixed with the BaTiO3-based material powder of a ceramic composition to prepare slurry. Subsequently, by a doctor blade method, for example, a green sheet of the dielectric ceramic composition is prepared from the slurry. Then, conductive paste including metal powder for forming an inner electrode is printed on the green sheet. A plurality of green sheets formed in this way and each having conductive paste printed on the obverse surface are so laminated that the conductive paste and the green sheet are alternately positioned and bonded under pressure. Subsequently, the laminated product is baked at a predetermined baking temperature to be integral (baking step). In the baking step, the ceramic composition in each green sheet sinters to form a ceramic dielectric layer, and metal powder in the conductive paste sinters to form inner electrodes. Thereafter, a pair of outer electrodes, each of which is electrically connected to a predetermined group of inner electrodes, are formed on surfaces of the laminated product.
In the above-described baking process, for the BaTiO3-based ceramic composition to properly sinter and exhibit good dielectric characteristics in the multilayer ceramic capacitor, the ceramic composition needs to be baked at high temperatures of about 1150-1350° C. On the other hand, as the metal for forming the inner electrodes, a metal material needs to be used which has a melting point that is higher than the baking temperature of the baking process, which can be baked at the same baking temperature as the ceramic composition, and which is not substantially oxidized at the high temperature in the baking process. As the metal materials that satisfy such conditions, Pd, Pt or alloys thereof are known. However, these metal materials are expensive, and hence, are not preferable. When such metal materials are used as the inner electrode material, the cost for the electrode material unfavorably increases as a larger number of layers are stacked to increase the capacitance of the multilayer ceramic capacitor.
Therefore, as the inner electrode material, the use of Ni and Ni alloys have been considered that are relatively inexpensive, have a low specific resistance and a melting point higher than the sintering temperature of the BaTiO3-based ceramic composition, and can be baked at the same temperature as the ceramic composition. However, in the baking at high temperatures in an atmosphere containing oxygen, i.e., in the air, for example, Ni isoxidized and the function as the electrode maybe lost. Further, nickel oxide maybe taken into the ceramic composition to deteriorate the properties of the capacitor.
When the baking step is performed in a reducing atmosphere or a low-oxygen atmosphere to prevent the oxidation of Ni, the BaTiO3-based ceramic composition is reduced to change the valence of Ti from 4 to 3. As a result, the composition becomes a semiconductor, and the insulating property is deteriorated. Further, the baking step in a reducing atmosphere or a low-oxygen atmosphere increases the oxygen vacancy in the BaTiO3-based ceramic composition, so that the life of the ceramic composition (time before insulation deterioration occurs) tends to be shortened.
Therefore, as a reduction-resistant BaTiO3-based ceramic composition in which properties deterioration such as a decrease in insulation resistance is less likely to occur even in the baking in a reducing atmosphere, a composition in which the molar ratio of BaO/TiO2 is no less than 1 or a composition in which part of Ba is replaced by Ca have been developed. Such ceramic compositions are disclosed in JP-A-S55-67567, for example.
On the other hand, due to the development of electronic devices having a small size, multifunction and high performance, a small size and a large capacitance are demanded for a capacitor which constitutes an electric circuit to be incorporated in such electronic devices. To satisfy the demand, in addition to the improvement of the dielectric material, the thickness of the dielectric layer between electrodes tends to be reduced to enable the lamination of a larger number of layers. However, to properly reduce the thickness of the dielectric layer, the ceramic composition constituting the dielectric layer needs to have a sufficient insulation resistance, and the deterioration with time of the ceramic composition needs to be sufficiently small. Further, in accordance with the size reduction, increase of functions and improvement of performance of an electronic device, its electric circuit has high density and is likely to be heated up during the use of the device. Therefore, for the ceramic composition constituting the dielectric layer of a capacitor in the electric circuit, it is demanded more strongly than before that the properties of the composition do not change largely due to the temperature change.
Such enhancement of properties and reliability is also demanded with respect to the reduction-resistant BaTiO3-based ceramic composition, and the enhancement of properties and reliability by adding various oxides have been studied. For example, such ceramic composition is disclosed in the Patent Documents 1-5 described below.
However, in prior art BaTiO3-based ceramic composition, it is difficult to satisfactorily enhance the insulation resistance, suppress the deterioration with time of the insulation resistance and suppress the capacitance change relative to temperature change while ensuring sufficient reduction-resistance. Therefore, with the prior art technique, it is difficult to enhance the properties of a multilayer ceramic capacitor utilizing Ni or Ni alloy as the inner electrode material to a level equal to or higher than that of a multilayer ceramic capacitor having Pd inner electrodes.
Patent Document 1: JP-A-S61-36170
Patent Document 2: JP-A-H06-5460
Patent Document 3: JP-A-H06-342735
Patent Document 4: JP-A-H08-124785
Patent Document 5: JP-A-H09-171937